Oh, so I can call it anything, lol

From what I've seen, the only difference between each one is the number of slots.

From what I've seen up to now, I've noticed the titles differ by a few offers:
A) The level of quality applied to the finished art pieces
B) The starting price & ongoing cost of said pictures
C) The level of supply & demand (how fast the pics are expected to be completed, how many interested commissioners flock to order, etc.)

Also, it seems like the "material"-based titles depend on the overall artwork delivery in and of itself.
Iron being a lesser material, thus resulting in simple basic sketches or monochrome-color shaded pics; bronze being a few steps up, usually including more vivid colored works & additional shading/backgrounds; steel being practically full-fledged commissions or other miscellaneous demands (ref.sheets, comics, etc.), that kind of thing.

At least... that's how I THINK it all works! ;)

Sounds legit to me n_n Does the number of slots differ?

Most likely the overall allotted amount of slots can correlate with whatever you feel most comfortable with. It could start easy, around 25 or maybe even 30, then grow and adapt over time to a higher amount. 50, 100, 150, etc.
The most slots I've ever seen a single artist work up to with this "Iron Artist" concept was 300. (!!!)

The name doesn't matter, what it is is an artist opens a certain number of slots (usually high volume depending on the artist) sometimes for a reduced price sometimes for full price, and goes through the slots one by one as often as possible to get the commissions out quick. It's just a sort of scheme to make A.) quick money or B.) get past art block/challenge one's artistic abilities OR both.

Alright. All of the above are correct for online things at the present time, but what is lacking is the origin of the term and the convention methodology.

Based upon the concept of the Japanese TV show "Iron Chief", artists are given one hour to utilize the secret ingredient that is revealed by the chairman of the event.

I first saw Iron Artist at Further Confusion #1 all those years ago. The creativity that can be applied to such diverse materials as cake, sewing thread spools, and the likes is absolutely amazing.

Interesting, I did not know that.

Like everyone here has said, it's kind of meandered away from its original meaning. I think you could make it whatever you wanted to at this point.